1. Field of the Invention
The present invention relates to an internal combustion engine with multiple intake valves for each cylinder and an improved exhaust gas recirculation (EGR) arrangement.
2. Description of the Related Art
In a known internal combustion engine, two intake valves are arranged in respective intake passages for each cylinder, and it is possible to improve the performance of the engine by constituting one of the intake passages by a helically shaped intake port and the other by a straight shaped intake port. For example, Japanese Unexamined Utility Model Publication No. 61-122362 discloses an internal combustion engine with multiple intake valves in which a flow control valve is arranged in the straight shaped intake port to close same to generate a strong swirl in the combustion chamber by a flow of intake air through the helically shaped intake port when the load of the engine is low. A fuel injector is arranged in the straight shaped intake port between the intake valve and the flow control valve, and an EGR passage is connected to this straight shaped intake port between the intake valve and the flow control valve. This arrangement is adapted to stratify fuel so that a richer mixture is distributed near the top of the combustion chamber and a leaner mixture near the top of the piston.
In this arrangement, the stratification of fuel-rich mixture near the top of the combustion chamber is established because a dead volume is formed in the straight shaped intake port between the intake valve and the flow control valve when these valves are closed. Fuel stays in that dead volume in the straight shaped intake port, is mixed with the EGR gas when the flow control valve and the intake valve are closed, and is then carried slowly by the flow of EGR gas into the combustion chamber when the intake valve is open while the flow control valve is closed. In this manner, fuel is supplied slowly into the combustion chamber, to generate a stratification of a fuel-rich mixture and the EGR gas near the top of the combustion chamber, with less interference by intake air flowing through the helically shaped intake port. This stratification is advantageous in effecting a good combustion at a low engine load even with a leaner air fuel ratio. Nevertheless, in this arrangement, fuel tends to be deposited on the bottom wall of the straight shaped intake port, since the actual flow of EGR gas is small, causing a delayed acceleration response when the flow control valve is open due to a change of the engine load, in which an increased amount of fuel must be carried by the intake air into the combustion chamber but the speed of the intake air instantaneously drops to fill the dead volume when the flow control valve is open.
Japanese Unexamined Utility Model Publication No. 61-126048 discloses a similar type of an internal combustion engine with multiple intake valves arranged in respective intake ports and a flow control valve. The second intake valve is open later than the first intake valve. In this case, two separate surge tanks are provided, with the first helically shaped intake port of each cylinder being connected to the first surge tank and the second straight shaped intake port of each cylinder being connected to the second surge tank. The flow control valve is arranged in the second surge tank to close the second straight shaped intake ports of all cylinders together, and a dead volume is constituted on the downstream side of the flow control valve, as in the previous case. Also, a fuel injector is provided in the second straight shaped intake port and an EGR passage is connected to the second surge tank. A leak passage is provided in the second surge tank, bypassing the flow control valve to allow a small amount of intake air to normally flow even when the flow control valve is closed, and this small amount of intake air, in addition to the EGR gas, carries fuel from the dead volume into the combustion chamber to generate a stratification of a fuel-rich mixture near the top of the combustion chamber when the flow control valve is closed, as in the previous case. Nevertheless, a similar problem of a delayed acceleration response arises in this case also.
Japanese Unexamined Utility Model Publication No. 61-110837 discloses an internal combustion engine with multiple intake valves arranged in respective intake ports. A flow control valve is arranged in the second intake port and a fuel injector is arranged in the first normally open intake port. In this case, the valve timing of the first and second intake valves is varied; i.e., the first intake valve is closed before the second intake valve, so that the compression ratio is increased by the earlier closing first intake valve when the flow control valve is closed at an engine low load. The compression ratio is reduced, but the inertia supercharging effect is increased, by the later closing second intake valve when the flow control valve is open at a high engine load.